Photoplethysmography is a measurement technique that uses light pulses from different portions of the electromagnetic spectrum (i.e., relatively narrow band light pulses having different center wavelengths) to quantitatively and noninvasively measure various blood analytes in a test subject. The measurement is based on the difference in the light transmittance of the various blood analytes (such as oxyhemoglobin and reduced hemoglobin) of the test subject at the different wavelengths. One type of photoplethysmographic process, known as pulse oximetry, is used to measure the oxygen saturation level of a person's arterial blood. In a pulse oximetry system, pulses of light having different spectral contents and center wavelengths are directed into the tissue under test, such as a finger or an earlobe. Transmitted light pulses are then received on the other side of the tissue under test (or in the case of a reflectance measurement, a small distance away on the same side) and are processed by the pulse oximetry system to determine the oxygen saturation level of the person's arterial blood.
Pulse oximetry systems generally include a main test unit housing, among other things, the control circuitry for the system; a probe unit for attaching to the appropriate tissue during a test; and signal transmission means for delivering signals between the main test unit and the probe. The main test unit may comprise either a benchtop unit or a handheld unit and, as described above, generally includes the control circuitry for controlling the operation, timing, and display functions of the system. The main test unit may also include a display for displaying measurement results to a user, an input device allowing a user to set various monitoring parameters, and an audible alarm for, among other things, warning a user of potentially dangerous analyte levels.
The probe unit generally comprises means for directing light from various emitters into the tissue under test and means for receiving transmitted light from the tissue. The probe may also include means for converting the transmitted light into an electrical signal for processing purposes. Because medical applications require the maintenance of hygienic conditions, it is often desirable that probes be disposable. Disposable probes should be relatively inexpensive to produce. In addition, because a probe is in contact with a person's skin during operation, it is important that the probe remain relatively cool during operation to prevent burns. Further, because the path a light signal takes through the tissue under test can affect the accuracy of the measurement it is important that all light pulses be directed into the tissue from essentially a single point. Lastly, it is generally desirable that probes be relatively small and lightweight so that they are easier to handle and remain in a fixed position on the appendage throughout testing without the tendency to fall off.
The signal transmission means generally includes both means for delivering signals to the probe for use by the probe in directing light into the tissue under test and means for delivering a signal representative of the light transmitted through the tissue under test back to the main test unit. It is generally desirable that the signal transmission means be as lightweight and flexible as possible. Therefore, it is advantageous to reduce the number of signal carriers in the signal transmission means. In addition, because the signal transmission means must be replaced periodically, it is desirable that it be relatively inexpensive to produce.